As the practical use of optical communication systems has been promoted in these days, optical communication systems are required to have a large amount of communication capacity and multi-functions such as switching, dividing, coupling, etc. of optical signals. In such an optical communication system, an optical switch is used for the changing-over of optical transmission lines and the switching of a network. In particular, an optical matrix switch, which comprises optical waveguide switches which are integrated to be positioned in an optical circuit composed of optical waveguides on a substrate, is a key device for the change-over and the switching of optical signals. This optical matrix switch is required to have a property of low loss in the application to a practically used optical communication system. For this purpose, an optical circuit including optical waveguides is required to have a property of low loss.
One type of an optical circuit has been described in "the technical report, OQE 88-147, 1988 of the Institute of Electronics Information and Communication Engineers". In fabricating this optical circuit, Ti is diffused into a Z-cut LiNbO.sub.3 substrate to provide belt-shaped regions of a higher refractive index than that of the substrate by a predetermined pattern, so that single mode optical waveguides are obtained to have intersections on the substrate. The optical waveguides are made to be proximate by a distance as small as several .mu.m, so that optical directional couplers are provided thereon.
In this report, it is explained that losses of optical signals are different among optical waveguide transmission lines, and the difference of the losses is dependent upon the number of intersections provided for each optical waveguide transmission path. The intersecting loss is reported to be approximately 0.35 dB for TM polarization at an intersecting angle of 7.degree. (seven degrees), and the number of the intersections ranges from 0 to 14 for each optical waveguide transmission path. Therefore, the difference of approximately 5 dB occurs in transmission loss between the optical waveguide transmission paths having no intersection and 14 intersections. As a matter of course, this intersecting loss is required to be low, so that loss of this optical device is totally decreased.
This intersecting loss occurs due to a phenomenon that an optical signal propogated through an optical waveguide is converted at an intersection from single mode to multi mode, and is again converted after the intersection from multi-mode to single mode. More precisely, this mode conversion occurring at the intersection, and a coupling of a signal light propagated through an optical waveguide to another optical waveguide for forming an intersection provide the intersecting loss.
The reduction of this intersecting loss has been discussed on pages 553 and 554 of "Electronics Letters, Vol. 19, Nov. 14, 1983" by A. Neyer. In this report, the intersecting loss is decreased by providing intersections each having a refractive index higher than that of optical waveguides.
However, this structure has a disadvantage in that it is impossible to provide the intersections having the higher refractive index simultaneously with the formation of optical waveguides of a substrate, so that a step of providing the intersections having the higher refractive index is additionally carried out, and in that it is required to control a refractive index of intersections with a high precision.
Another discussion has been described on pages 730 and 731 of "Electronics Letters, Vol. 25, Nov. 11, 1989" by K. Aretz et al. In this report, it is proposed that waveguides are tapered in a crossing area to reduce crosstalk and loss.
However, this proposed structure has a disadvantage in that the reduction of the intersecting loss is not realized at an intersecting angle of more than 7.degree. (seven degrees) which is ordinarily adopted in an optical device, so that it can not be applied to an ordinary optical device, although the effect is obtained at an intersecting angle of less than 6.degree. (six degrees). Furthermore, additional loss occurs in the tapered waveguides, so that a property of largely decreased loss is not obtained.
A still further structure of intersections has been described in "International Topical Meeting on Photonic Switching, Technical Digest, PD 13-1, Mar. 1, 1989" by T. O. Murphy et al. In this report, an intersection having a modified structure of optical waveguides is proposed.
However, this structure has a disadvantage in that its configuration is complicated, and it is necessary to define an optimum structure for each intersecting angle. As a result, it is difficult to design and fabricate an optical waveguide circuit.